This invention relates to valve actuating mechanisms and, more particularly, to desmodromic mechanisms wherein the actuator is moved positively in both opening and closing directions by mechanical action of the mechanism.
U.S. Pat. No. 5,937,809, issued Aug. 17, 1999, discloses cam driven variable valve timing (VVT) mechanisms which are relatively compact, and are applicable for operating individual or multiple valves. In these mechanisms, an engine valve is driven by an oscillating rocker cam that is actuated by a linkage driven by a rotary eccentric, preferably a cam. The linkage is pivoted on a control member that is, in turn, pivotable about the axis of the rotary cam and angularly adjustable to vary the orientation of the rocker cam and thereby vary the valve lift and timing. The rotary cam may be carried on a camshaft. The oscillating cam is pivoted on the rotational axis of the rotary cam.
The engine valve driven by the cam is conventionally provided with a return valve spring that closes the valve as the oscillating cam is returned to a valve closed position. The linkage driven by the rotary cam is not directly acted upon by the valve spring and so is provided with one or more return springs for returning the linkage to the valve closed position as the rotary cam is rotated, so that the cam follower returns to the cam base circle (the valve closed position). Further development of these mechanisms has included design of improved return springs for biasing the actuating linkage toward the valve closed position. Additionally, desmodromic cam driven mechanisms have been developed for positively driving the oscillating cam and linkage in both the opening and closing directions. Such mechanisms include both opening and closing cams driving the linkage and thus eliminate the biasing springs for the linkage, but not for the valve.
A desmodromic mechanism allows the oscillating cam and linkage to be operated at higher engine speeds without floating, that is, without losing contact of the follower with the opening cam. However, some amount of lash must be provided in the desmodromic mechanism to avoid overstressing the components by interference of the mechanism""s opening and closing portions. Because excessive lash can cause noise and stresses due to accelerations of the moving components, suitable means for taking up the lash in the oscillating cam drive mechanism have been considered.
The present invention provides a valve actuating mechanism intended for use with a desmodromic cam driven variable valve timing mechanism but applicable also to other desmodromic cam and rocker devices.
In an exemplary embodiment, the mechanism includes opening and closing rotary cams rotatable together about a primary axis and a rocker pivotable about a first pivot axis spaced from the primary axis. The rocker is connectable to actuate a valve by oscillating motion about the pivot axis and has first and second rocker arms respectively carrying opening and closing cam followers spaced from the pivot axis. The cam followers operatively engage the opening and closing rotary cams respectively and are operable by the cams for positively oscillating the rocker in either direction about the first pivot axis;
The closing cam follower has a pad engagable with the closing cam and carried by a hydraulic lash control piston slidable in a bore of the second rocker arm. The bore is connected with a pressure oil supply including a check valve preventing reverse flow of oil toward the oil supply. A bleed orifice in the pad is sized to allow restricted outflow of oil from the bore to avoid excessive loading of the opening and closing portions of the mechanism, the piston being advanced by the oil pressure to take up lash occurring between the cams and cam followers.
The rocker as described above could be connected to directly actuate a valve in opening and closing directions and the lash adjusting means would operate to take up lash in the manner indicated. However, the rocker may also be connected as part of an oscillating cam variable valve timing (VVT) mechanism including, for example, an output lever connected to the first rocker arm and carrying an oscillating output cam engaging a valve actuating member, and a control member pivotable about the primary axis and carrying the rocker on the first pivot axis. Moving the control member angularly varies engagement of the oscillating cam with the valve actuating member between full lift and no lift valve opening positions, thus providing variable lift and timing of the valve.
These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings.